GNS Science

Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)

New Zealand Cenozoic Mollusca

Spissatella subobesa (Marshall & Murdoch, 1919)


(Pl. 11a): GS10837, I40/f001, head of Lake Waitaki, Duntroonian (GNS)

Beu & Maxwell (1990): Chapter 10; p. 146; pl. 11 a.

Synonymy: Crassatellites subobesus Marshall & Murdoch 1919, p. 257

Classification: Crassatellidae

Description: Rather large for genus (length 43-65 nun), relatively elongate (length/height ratio 1.48-1.70), moderately inflated; posterior area concave, differentiated by well-defined umbonal ridge. Lunule well developed on each valve, deeply impressed; escutcheon prominent and deeply impressed on right valve, much narrower and more subdued on left valve. Commarginal sculpture on flanks of low, narrowly rounded ridges, 21-24 per cm at 1 cm from apex, in some specimens weakening distally; ridges much finer and more numerous and tending to be subobsolete on posterior area. Left hinge with narrow anterior lateral tooth formed by end of lunule, thin anterior and median cardinals, and a posterior cardinal tooth which is scarcely distinguishable from escutcheon margin; posterior lateral socket well defined. Resilifer deeply impressed, reaching almost to hinge margin. Right anterior cardinal tooth coalescent with edge of lunule, median cardinal narrowly triangular. Posterior lateral tooth finely transversely striate, coalescent with escutcheon. Adductor muscle scars oval, subequal. Internal margins smooth.

Comparison: Spissatella subobesa is distinguished from S. trailli (Pl. 18 i, k) by its larger size, its typically more elongate shell and its considerably finer commarginal sculpture. Some individuals approach topotypes of S. trailli in shape but can be distinguished by their finer sculpture, particularly on the umbones. S. occulta (Duntroonian, Wharekuri) is much less elongate and less strongly inflated than S. subobesa, has a less well defined umbonal ridge and a slightly convex rather than a straight or slightly concave posterodorsal margin. The two Wharekuri species apparently do not occur together in the Wharekuri Greensand and presumably were ecologically segregated. Yet another Duntroonian species, S. poroleda (Shell Gully, Chatton) is more like S. subobesa in overall shape, but is less strongly inflated, has a more strongly convex ventral margin, and has a sharp rather than a rounded umbonal ridge. "Spissatella" scopalveus Finlay, 1926b [p. 258], a feebly sculptured species assigned by Finlay to Spissatella, is more inflated than Spissatella species, and seems to be a small species of Eucrassatella, resembling several of the smaller Australian species revised by Darragh (1965) rather than other New Zealand fossils.

Distribution: Duntroonian; Wharekuri Greensand, Waitaki River, opposite Wharekuri (type); head of Lake Waitaki (common).

Cite this publication as: "A.G. Beu and J.I. Raine (2009). Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990). GNS Science miscellaneous series no. 27."
© GNS Science, 2009
ISBN 978-0-478-19705-1
ISSN 1177-2441
(Included with a PDF facsimile file copy of New Zealand Geological Survey Paleontological Bulletin 58 in CD version from: Publications Officer, GNS Science, P.O. Box 30368 Lower Hutt, New Zealand)


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